Documentation is the key to making one's knowledge useful to others. Without it, information is needlessly lost and life is made harder unnecessarily.Software
Documentation is key to software development. Call it tedious, call it boring, call it a waste of time, call it whatever you want. The minute you have to explain the architecture of your application or the minute someone else has to fix a bug in your program or the minute you ask someone else to help you with your program is the minute that makes all the work of documentation worth its weight in gold.
What good is an excellent idea when the idea is not shared? What makes you think that you will not want to understand this code one year from now? What makes you think you will understand all the nifty tricks you performed without any comments in your code?Hardware
The Tesla coil has been constructed enough times that if even 0.1% of such projects were well documented, one could find much more advice and techniques to building them than one can today. What we have here is a return on investment problem -- nobody wants to put in the initial effort to document a project when one can simply learn how to do it, then go on to the next thing.